On 15/06/2016 13:10, John Willis wrote:
So SAC scale and being outside a park polygon/relation is good enough
to allow a data consumer and the folks over in -carto to render a
"footway" and a "trail" differently and reliably enough? What happens
when I have a strong mix of =pedestrian, =footway, and ="trail"? In
the same park area?
Before answering that, let's take a step back and have a think about
what we're trying to do here:
o People tag stuff so that the essential nature of what's on the ground
is capture. That might be "is mainly designed for use by foot traffic"
(highway=footway), "has a gravel surface" (surface=gravel) or "is part
of some sort of route relation" (appropriate relation membership). Data
consumers occasionally grumble about specific tagging choices, but
usually if the data's there, they can somehow deal with it.
o Renderers have a limited number of ways that they can render stuff
without things becoming _way_ too complicated. If you look at osm-carto
you can see that for linear features such as roads and tracks it uses:
1) feature width
2) feature colour
3) casing width
4) casing colour
5) How it's actually rendered (continuous line, dots, dashes, whatever)
6) Some text printed alongside the feature
Those are basically the degrees of freedom you've got.
"osm-carto" uses 6 for names, and chooses to use just lines for
footpaths and tracks (we'll leave highway=pedestrian out as it's
rendered - correctly in my view - as a road), which basically leaves you
with 1, 2 and 5. Of those, the combination of 1 and 2 need to be
carefully adjusted together so that a particular feature has the right
level of "importance" in a particular rendering at a particular zoom
level. "osm-carto" de-emphasises footways at the expense of other
features; other styles (see the two examples below) emphasise them more.
Of the available choices "osm-carto" currently uses 1 and 2 to
distinguish between footway, bridleway, cycleway and track (and I
believe puts "path" in one of the first two buckets based on other
tags). It uses 5 to show paved vs unpaved (see
http://www.openstreetmap.org/#map=18/53.21858/-1.49726 ). It doesn't
show relation membership.
Other styles do things slightly differently. Thunderforest Outdoor
doesn't show the surface difference, de-emphasises roads but does show
relation membership:
https://c.tile.thunderforest.com/outdoors/15/16247/10641.png
I use a style based on OSM-carto that doesn't show surface explicitly on
these features (or relation membership) but does show England and Wales
legal status (using colour) and track width (using dashes instead of dots):
http://i.imgur.com/sjytRiy.png
So data consumers have choices, and there's a limit to the differences
in the data that they can render. If someone asked me 'to render a
"footway" and a "trail" differently' my first question would be, apart
from tags that I already understand such as surface, tracktype,
smoothness, etc., what's the difference? In my view if there's a muddy
flat 1m-wide path in a city centre and a muddy flat 1m-wide path miles
away, then they should be rendered the same - apart from relationship
membership of course, if a style renders that.
It seems to me - as a person who is a Kountry Kilometer away from
being data consumer - that using a subtag or similar to let mappers
tag trails and other rough footways (the "track" end of footway) is a
much more straightforward and direct solution to get trails to render
differently than more casual and easily traversed footways found in a
city park or rose garden.
I am really having trouble understanding the reasoning behind the
resistance when it removes uncertainty and confusion while tagging.
Clearly there is confusion about tagging these sorts of features (this
disussion wouldn't exist if there wasn't) but I'm really struggling to
see any difference between a "footway" and a "trail" that can't be
expressed in other, more frequently used tags. To take a random
example, what should I infer about
http://www.openstreetmap.org/way/372458198 ? How wide is it? What
surface does it have? What are the access rights for various sorts of
traffic?
Cheers,
Andy
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